The biggest problem is obviously that omni diet is entrenched and promoted by our societies.
In India, for example, vegetarianism is very common due to it being pretty normal as a custom. Even many meat eaters eat vegetarian meals mostly. Meanwhile, in Canada, meat is highly subsidized and because of that you can get a McDouble for like $2, the real cost of which is a lot higher.
People don't want to go through the effort of changing their diet because in all honesty it is a big change. But I don't know how they justify that to themselves considering all leftists know the catastrophic environmental cost of large scale meat production and the cruelty that comes with it.